Nitrogen Application Increases the Productivity of Perennial Alpine Cultivated Grassland by Improving Soil Physicochemical Properties and Microbial Community Characteristics

Nitrogen fertilizer application has significantly contributed to the overall improvement of grassland productivity. However, the effects of different nitrogen forms in cultivated grasslands and the mechanisms by which nitrogen addition improves plant productivity remain unexplored. In this study, four nitrogen application rates were assessed using urea as the nitrogen source (CK: 0 kg N hm−2 y−1, U1: 22.5 kg N hm−2 y−1, U2: 45 kg N hm−2 y−1, and U3: 67.5 kg N hm−2 y−1), as well as three nitrogen forms (amide nitrogen: urea, U3; ammonium nitrogen: ammonium sulfate, A; nitrate nitrogen: calcium nitrate, N) at 67.5 kg N hm−2 y−1. Aboveground biomass increased gradually with increasing N application, being 12.02